1. Field of the Invention
The present invention relates to a scanning type projection exposure apparatus and a device production method using the same. Scanning type projection exposure apparatuses are used in the lithography step carried out to produce, for example, semiconductor devices (including an integrated circuit (IC) or a large scale integrated (LSI) circuit), or image pickup devices (such as charge-coupled devices (CCD)), or display devices (such as liquid crystal panels), or magnetic heads. They are suitable for projecting a pattern on a reticle (mask) onto a wafer by a projection optical system with a uniform exposure amount.
2. Description of the Related Art
In recent years, semiconductor devices, including an integrated circuit (IC) or a large scale integrated (LSI) circuit, have rapidly become increasingly integrated. This has resulted in the production of various types of projection exposure apparatuses which are important devices when micromachining semiconductor wafers. They include 1xc3x97 magnification projection exposure apparatuses (mirror projection aligners), and reduction projection exposure apparatuses (steppers). In mirror projection aligners, a photosensitive substrate or a wafer (coated with a photosensitive agent), and mask (with a circuit pattern) are exposed by scanning them, using a 1xc3x97 mirror optical system (with a circular exposure area). In reduction projection exposure apparatuses, an image of a pattern of a mask (reticle) is formed on a wafer by a refractive optical system, and the wafer is exposed by the step-and-repeat method.
In recent years, there has been a trend towards forming larger chip patterns on one semiconductor device. This has given rise to the demand for projection exposure apparatuses which can project a larger area pattern of a mask onto a wafer by exposure.
To respond to this demand, various types of scanning type reduction projection exposure apparatuses which utilize the step-and-scan method have been proposed. They are capable of providing high resolution and allow larger screen sizes to be used. In these exposure apparatuses, a pattern of a reticle is illuminated by light beams which have been passed through slits. The illuminated pattern is projected onto a wafer by exposure as a result of scanning operations performed through a reduction projection optical system.
The various types of scanning type reduction projection exposure apparatuses proposed include the following two types. In one type, which is an improvement over the 1xc3x97 magnification scanning type exposure apparatus comprising a reflective projection optical system, a refractive element is incorporated in the projection optical system, and combined with a reflective element. In the other type, a reduction projection optical system which comprises only a refractive element is used in order to scan a mask stage and a wafer stage in synchronism with each other at the velocity ratio based on the reduction magnification.
In general, in order to obtain a high resolution pattern, it is necessary to uniformly expose a wafer with light.
A method of adjusting the amount of light exposure to a uniform amount with respect to a wafer (which is irradiated) in a 1xc3x97 scanning type projection exposure apparatus is disclosed in, for example, Japanese Patent Laid-Open No. 62-193125. In the disclosed method, when the illuminance at the coordinates in a direction perpendicular to the scanning direction is high, the irradiation range (or illumination area) in the scanning direction is narrowed. On the other hand, when the illuminance at these coordinates is low, the irradiation range is widened. By adjusting the irradiation range in the above-described way, scanning and exposure operations can be performed such that the integration light amount for the coordinates in the scanning direction becomes uniform.
In scanning type projection exposure apparatuses, a portion of the light within the illuminating system is separated. Then, the light portion amount is detected by a detector, and the detection result is used so that a suitable amount of light can be supplied on a wafer surface. In this case, it becomes very important to previously determine the relationship between the illuminance at the wafer surface and the amount of light entering the detector, and to precisely monitor the illuminance at the wafer surface, from the amount of light entering the detector.
However, in scanning type projection exposure apparatuses, the exposure amount at each point on the wafer surface depends on the integration light amount in the scanning direction. Therefore, when, in order to adjust the exposure amount in the non-scanning direction to make it uniform, width adjustments in the scanning direction are carried out for every location in the non-scanning direction, and changes occur in the illumination area width in the scanning direction at a location where the amount of light falling thereupon has been measured by a detector, and the relationship between the measured amount of light and the integration light amount, falling on the wafer surface, changes. Consequently, the exposure amount cannot be controlled precisely.
Accordingly, it is an object of the present invention to provide a scanning type projection exposure apparatus which can control the exposure amount precisely. In the scanning type projection exposure apparatus, an illumination area width in the scanning direction can be changed in order to make uniform the exposure amount in a direction perpendicular to the scanning direction.
To this end, according to the present invention, there is provided a scanning type projection exposure apparatus comprising:
means for holding a mask with a pattern formed thereon;
means for holding a wafer, onto which the pattern on the mask is transferred;
wherein while the mask and the wafer are being scanned by means of the mask holding means and the wafer holding means, the pattern is illuminated such that an illumination area narrower than the pattern is successively illuminated in order to transfer the pattern onto the wafer;
an illumination optical system for illuminating the mask, the illumination optical system comprising a slit for determining the width of the illumination area in the scanning direction, the slit having an area whose width does not change in the scanning direction, the slit being provided for changing the width in the scanning direction of the illumination area, excluding the area whose width does not change;
a first light detector for extracting a portion of the light of the illumination optical system in order to detect the intensity of the light portion;
wherein the exposure amount with respect to the wafer is controlled based on an output of the first light detector; and
wherein the first light detector detects the intensity of the light at a position located in correspondence with the area whose width does not change.